Avulsion fractures of the tibial tuberosity occur mainly during sport activities and are closely related to the strains exerted on the anterior tibial tuberosity by the extension complex of the thigh. A knowledge of the mechanical aspects of these avulsions may improve understanding of the mechanisms of such injuries. In such avulsion fractures, tensile forces due to the contraction of the quadriceps complex overcome the cohesive forces within the apophyseal cartilage. A 1-month cast immobilisation on an extended knee gives good results in the management of nondisplaced fractures. In minor displacements, such immobilisations follow closed external reductions. Open reductions and stable screw fixations precede a 3-week immobilisation for displaced fractures. Long term results are regularly good in well-managed cases.